A new classical collective paramagnet: the monopole liquid

D. SLOBINSKY; R. BORZI

Bariloche

Workshop; Exploratory Workshop on Condensed Matter Physics; 2017

A monopole liquid is a magnetic charge-disordered spin system defined over an Ising pyrochlore lattice, with one single topological charge or monopole in each tetrahedron. We define a simple model Hamiltonian for this system (which is a spin liquid, as well as a liquid of monopoles) and compare its thermodynamics with that of spin ice, a phase free of these charges. We find that our system of monopoles presents a larger residual entropy, and encompasses two further spin liquids: a polarized monopole liquid, for moderate magnetic fields along [100], and the monopole crystal, a fragmented spin liquid with no analog within spin ice. As previously guessed, a field along [111] leads to a three-dimensional Kasteleyn transition. We find that it is of a new type, with excitations (in the form of loops of reversed spins) existing even below the transition temperature. In other words, there exist a whole nondegenerate manifold (the loop phase) associated with the zero-string topological sector. We briefly discuss how the different monopole liquid states can be related to previous experiments and models, or stabilized by the combination of magnetic interactions and lattice deformation, and show how other interactions present in real materials may enrich its properties.